Biaxially oriented polyester films (e.g. biaxially oriented polyethylene terephthalate=BOPET) are increasingly used as packaging material, e.g. for food or drink, for pet food, or for detergent. Their characteristic properties for this application sector are their high optical transparency, their high gloss, and their high mechanical strength. The films mentioned also feature good barrier properties, in particular with respect to transmission of oxygen. In many applications, it is desirable for the film to provide a barrier better than that provided by BOPET. Examples here are the packaging of cheese, of coffee, of herbs and spices, or of baby foods, to mention just a few examples.
In most cases, BOPET is processed to give a laminate with other polymers (PE, PP), with paper/cardboard, or with other films or foils (e.g. BOPP, aluminum). To produce this laminate, the PET film is often extrusion-coated, using polyolefins, e.g. PE, PP, or ethylene-propylene copolymer. This is a technically simple way of obtaining the desired properties of the laminate, e.g. hot-sealability, good adhesion to other materials (e.g. to aluminum foil), or additional desired stiffness via the thick PE layer applied by extrusion (PE=polyethylene, PP=polypropylene).
However, a PET surface which has not been pretreated is not receptive to extrusion coating with polyolefins. In conventional extrusion coating, the processor will corona-treat the PET film supplied by the producer, coat the corona-treated film with an adhesion-promoting layer, dry the layer, and then extrusion-coat the material with another polymer. The necessity of coating the film with an adhesion-promoting layer implies an additional processing step, which demands expensive additional machinery and causes yield losses.
Biaxially oriented polyesterfilms which have a coating that promotes adhesion to various materials on at least one of their two surfaces are known in the prior art.
EP-A-0 359 017 describes an oriented film comprised of thermoplastic and having, on at least one of its two surfaces, an adhesion-promoting layer comprised of a hydrolyzed amino-functional silane, this layer being applied in the form of an aqueous dispersion to the film. The description of the adhesion-promoting layer says that it is comprised of a dried residue of a hydrolyzed aminosilane compound whose unhydrolyzed form has the following formula:(R1)aSi(R2)b(R3)cR1 is a functional group having at least one primary amino group, R2 is a hydrolyzable group selected either from short-chain alkoxy groups having from 1 to 8 carbon atoms or from an acetoxy group or from a halide, and R3 is an unreactive, non-hydrolyzable group, either a short-chain alkyl group having from 1 to 8 carbon atoms or a phenyl group. (a) here is greater than or equal to 1; (b) is greater than or equal to 1, and (c) is greater than or equal to 0, and the rule here is that a+b+c=4. The application weight at which this adhesion-promoting layer is applied to the PET film is such that it improves adhesion to polyolefins, applied via extrusion coating. Disadvantages of the film are its optical properties, such as transparency or gloss, and also its barrier properties, in particular with respect to transmission of oxygen. Because of these disadvantages, this film is not used in high-performance packaging applications (cheese, herbs and spices).
Transparent, biaxially oriented polyester films which feature improved barrier properties are likewise known from the prior art. In most instances, the films acquire their improved barrier properties off-line after the production process via a further processing step. Examples here are extrusion coating, coating or lamination with barrier materials, in-vacuo coating with metals or with ceramic substances, or plasma polymerization in combination with vacuum coating.
An exception here is the process described in more detail in WO 99/62694, in which a multilayer, coextruded polyester film which comprises at least one layer comprised of EVOH (ethylene-vinyl alcohol) is simultaneously biaxially oriented. This film features good mechanical properties, and in particular features good barrier properties with respect to transmission of oxygen. The best value given for achievable oxygen transmission OTR (oxygen transmission rate) in the specification are 5 cm3/(m2·bar·d). The disadvantage of the process, inter alia, is that regrind produced during the production process cannot be reintroduced into the production process without sacrificing the good optical and physical properties of the film.
Another exception is the biaxially oriented film described in JP 2001-001399, which is comprised of a mixture of polyethylene terephthalate and poly(m-xyleneadipamide) (MXD6). The proportion of poly(m-xyleneadipamide) (MXD6) in the film is from 10 to 40% by weight, and the corresponding proportion of polyethylene terephthalate is from 60 to 90% by weight. According to the invention, the film is simultaneously biaxially oriented. The specification gives the following data for the stretching parameters: The stretching ratios in both directions are from 2.5 to 5.0. However, in the examples the film is only oriented by a factor 3.0 in the machine direction and by a factor of 3.3 transversely to the machine direction. The overall stretching ratio is therefore 9.9. The stretching temperatures in both directions are from 80 to 140° C. In the examples, the film is stretched in both directions at 90° C.
According to JP 2001-001399, when a simultaneously oriented film is compared with a sequentially oriented film (e.g. oriented first in machine direction (MD or MDO) and then in the transverse direction (TD or TDO)), it has lower haze and gives more dependable processing, i.e. can be produced with a smaller number of break-offs in the second stretching phase (e.g. in the transverse direction). According to the above specification, the degree of crystallization that occurs during the sequential (non-inventive) orientation in the first stretching step (e.g. MDO) is so great that the film becomes cloudy during the second (subsequent) orientation process and becomes more delicate with respect to any further orientation process. According to the (comparative) Examples 3 and 4 set out in the specification, a polyester film with from 10 to 40% of MXD6 cannot be produced by the sequential process, because it tears in the second stretching phase.
The biaxially oriented films produced according to JP 2001-001399 by the simultaneous process feature low haze, but in particular feature good barrier action with regard to oxygen permeation. The film achieves an oxygen transmission OTR smaller than 30 cm/(m2·bar·d). According to the invention, the haze of the film is smaller than 15%. However, the film has a number of disadvantages:    It has a comparatively low level of mechanical properties. In particular, the modulus of elasticity and the ultimate tensile strength are unsatisfactory.    It tends to block and is therefore difficult to wind.    It has comparatively rough surfaces. The film also has a matt appearance, undesirable for many applications. It is therefore also comparatively difficult to print, to metallize, or to coat.